Conventional Ethernet cables employ eight wires, only four of which are used to carry communication signals. The other wires are available for other uses. One such use is to carry power from one Ethernet-connected device to another such device. This allows the device receiving the power to avoid the need to obtain power from a separate source.
Several standards have been developed for supplying power over Ethernet cables. One standard is the conventional 802.3af proposed by the IEEE, which supplies 48 volt power over at least two of the wires, using either of the two polarities, such as +48 volts or −48 volts. The IEEE is an organization composed of multiple member organizations that develop standards, among other things. Another standard for supplying power over Ethernet cables has been promulgated by Cisco Systems, Inc., and supplies −48 volt power. Cisco Systems, Inc., is a corporation that supplies communications equipment, including Ethernet processing equipment. The systems drawing such power are typically compatible with only one such power polarity. What is needed is a system and method that can supply power over an Ethernet cable using either of the polarities.